A parallel arcing fault occurs when an undesired electrical arc bridges the gap between two conductors or a conductor and ground. Since the dielectric strength of air is known to be approximately 31 kV/cm, it is generally understood that exposed conductors in air and at line voltages (e.g., 117V rms) must come to within a few mils (1 mil=0.001 in) of each other before an arc can strike (Note that 167 Volts peak divided by 31 kV/cm is 2.1 mils). Power distribution systems are therefore commonly designed to avoid this by maintaining conductor separation much greater than a few mils and/or providing adequate insulation between the conductors. It is also understood that parallel faults may develop if the separation between said conductors is inadvertently diminished or if the integrity of the insulation is violated as the result of, for example, chaffing caused by mechanical vibration. In addition to these obvious scenarios there are subtler, less obvious ways in which parallel faults might develop, particularly in the aircraft environment.
First there must be exposed conductors. These can be found at the terminals of circuit breakers, on terminal strips and at some connector terminals. Conductors inside wires may become exposed as a result of aging cracks or holes in the insulation. In October of 2000, an FAA aviation industry task force reported that during the inspection of a relatively small amount of Kapton wiring on both a >20 year old Airbus A300 and a Lockheed L-1011, 9 cracks that exposed the conductor were found on the former and 13 cracks on the latter. See Christopher Smith, Transport Aircraft Intrusive Inspection Report, The Intrusive Inspection Working Group (Dec. 29, 2000). From this limited data they extrapolated that there might be up to 900 cracks on the A300 and 3,000 on the L-1011. It should be recognized that even a large number of aging cracks in the wire insulation pose little danger of arcing unless the separation between two cracks or a crack and the airframe becomes small enough for an arc to occur at normal operating voltages. How small? Based on a simple linear extrapolation of the dielectric breakdown voltage of air the separation would have to be on the order of several mils or less for arcing to occur. Unfortunately, parallel faults can develop across separations substantially greater than this due to secondary environmental influences.
Once there are exposed conductors within a fraction of an inch of each other or the airframe, initial conduction across the gap can develop in several ways. First, if a voltage surge high enough to span the gap occurs, resulting from an inductive switching transient or perhaps induced by a lightning strike, the localized heating from the momentary arc can carbonize insulating material under the arc, including dust or other contamination on the surface of the wire, and form a high-impedance conduction path. A second and perhaps more likely scenario involves water that normally condenses on the inner shell of the aircraft as the outside temperature drops precipitously during flight. This condensation water readily dissolves impurities that are present and becomes a mildly conductive electrolyte that can conduct a small AC current across the gap. This current produces heat and the heat evaporates the water leaving behind molecular islands of salt that eventually form a kind of archipelago of larger conductive islands. Each time it gets wet, the electrolyte itself will support current flow and add more salts to the developing archipelago. The arc breakdown potential is proportional to the sum of the distances between conductive islands.
What happens next depends on the wire insulation material. Both a Polyimide plastic, sold under the brand name “Kapton”, and Fluorocarbon plastic, sold under the brand name “Teflon”, have been widely used in aircraft wiring. If the insulation is Teflon, this low-current arc will repeatedly extinguish with little damage done. If the insulation is standard Kapton (i.e., non fire-resistant), the low-current salt bridge arc will likely soon involve the Kapton itself, expand and escalate rapidly into a near explosion of current that often destroys not only the wires involved, but also adjacent ones in the wire bundle.
A number of articles in the press have noted the apparent arcing danger of Kapton insulation. In the presence of a low-current arc Kapton insulation can easily develop into an explosive arc while Teflon exhibits only gradual, slow deterioration of the Teflon heated by the arc. Research by the present inventor has shown that Kapton can become conductive in the near vicinity of an arc. Localized heating of the Kapton apparently oxidizes portions of it to a conductive intermediary (remaining amber in color, it does not appear to be completely oxidized to carbon at this stage) that reduces the arc gap and increases the electric field strength. Under a stereo microscope, one can see minute pieces of Kapton conducting current and arcs jumping from the glowing Kapton to the metal conductor. Insulating materials like Teflon contain halogens that inhibit oxidation by producing by-products that are more electronegative than Oxygen, e.g., Fluorine. Standard Kapton doesn't have this fire inhibitor and this may, in part, explain the difference.
The formation of aging or stress-induced cracks in the insulation and the repetitive condensation, wetting, and low-current arc induced evaporation cycle together form a progressive degeneration process that can lead to parallel arcing. Mechanical chafing of the wire insulation can also lead to parallel arcing. As the world's fleet of commercial and private aircraft age, particularly now that many aircraft over 20 years old are still flying, the likelihood of such faults occurring increases. If these developing faults can be detected early enough, the insulation could be repaired or replaced before the fault develops into a dangerous arcing fault. A need exists, therefore, for a means by which wiring harnesses could be tested to reveal these conditions as they are developing.
The present inventor realized that developing parallel faults, due to mechanical chafing or aging cracks in the insulation, for example, will generally exhibit a progressively declining breakdown voltage until a point is reached where the arcing becomes self-sustaining and dangerous. Such developing faults in the insulation are initially non-conductive and usually so small as to make no perceptible change in the characteristic impedance of the cables. The only practical way to reveal reduced conductor spacing (or a non-conducting salt bridge) is to apply a higher-than-normal voltage to the junction, a testing procedure commonly referred to as HiPot (High Potential) testing. A traditional DC HiPot tester, however, which allows 10 ma of current to flow after breakdown, can itself heat the insulator enough to form a carbon track and damage the insulation. A conventional HiPot tester can also damage equipment left connected to the harness during testing. A further need exists, therefore, for a means by which the breakdown voltage can be measured without damaging the wire insulation or any electronic devices inadvertently connected to the harness.
Wire harnesses in modern aircraft are dense, multi-legged, and routed throughout the plane—up to 140 miles of wire in a typical wide-body jet. Hundreds of connectors are placed along the harnesses to allow modular assembly and disassembly of components. Because access to wiring harnesses is very limited on an operational aircraft, such testing is probably best done during periodic heavy-checks, whereupon panels and floorboards are removed to facilitate access. Even in this case, however, specific wire bundles may be very long and difficult to access. There exists a further need, therefore, to provide practical means to physically locate the developing parallel fault once it has been revealed.